This is a particular passion of mine. All my boys will be doing at least level one outlined below. I think these skills are just so necessary to be able to communicate to someone what you want or need from an item in three dimensions.
My opinion of what needs to be covered:
First level (and almost essential IMO)
- plane geometry (possibly covered elsewhere if you have a good maths program), things like dividing a line of unknown length in to 5 even segments, or dividing an unknown angle evenly in half, proportionals, polygons, tangents, little tricks for doing things like drawing a line parallel a certain distance from a line you already know
- involutes, archimedian spirals, cycloids, epicycloids, hypocycloids (at least an understanding if not necessarily a strong knowledge)
- orthographic projection - at least first angle, ie plans, elevations, sections and an understanding of the different line styles used to represent hidden parts etc
- understanding of scales, tolerances and fits
- basic architectural drawing (just in case they ever choose to build or extend their home)
- basic isometric drawing (horizontal lines represented as 30 deg to right and left to show a 3D image)
- how to create either orthographic projections from isometric drawings and vice versa
- basic perspective drawing with only straight lines
Second level and beyond
- any of the above that will require circles and curves, could be hard for a fine motor skills challenged kid
- more complex orthographic projections of two solids intersecting and being able to show the shape of the cut into one to fit the other, think gutters intersecting and knowing the shape of the "hole" if the main shape is rolled out flat
- true shapes of truncated simple solids
I am currently looking into what text to use. This may take some research but I will get back to you on this.
PS Aha, found the text I used in a slighter younger version. Check out this one available through Booktopia in Australia.